Choose ripe but firm Bosc pears for best results. They hold their shape well during poaching. Use a good quality red wine vinegar for the vinaigrette, as its flavor will shine through. For a richer vinaigrette, you can add a teaspoon of honey or maple syrup to balance the acidity. The poached pears can be made a day ahead and stored in the refrigerator. If you don't have walnut oil, olive oil can be used as a substitute, though the flavor will be slightly different.